www.hackthissite.org is a free, safe and legal training ground for hackers to test and expand their hacking skills. More than just another hacker wargames site, they are a living, breathing community with many active projects in development, with a vast selection of hacking articles and a huge forum where users can discuss hacking, network security, and just about everything. Tune in to the hacker underground and get involved with the project.
I have tried it and completed some level, and I definitely enjoyed it. Hope you too!